1. Field of the Invention
This invention relates to high performance permanent magnets used in various electric appliances, and more particularly, to permanent magnets in the form of rapidly quenched alloy materials of Fe-R-B and Fe-Co-R-B systems wherein R is a rare earth element.
2. Prior Art
Typical of high performance rare earth magnets are Sm-Co magnets. They are mass produced by powder metallurgy and some exhibit a maximum energy product of as high as 32 MGOe. However, Sm and Co source materials are very expensive. Those rare earth elements having a relatively low atomic mass such as cerium, praseodymium, and neodymium are supplied in more plenty and thus less expensive than samarium. To take advantage of inexpensive iron, Nd-Fe-B magnets have been recently developed. Japanese Patent Application Kokai No. 59-46008 describes sintered Nd-Fe-B magnets, and Japanese Patent Application Kokai No. 60-9852 describes rapid quenching of such magnets. The conventional powder metallurgy process for the manufacture of Sm-Co magnets can be applied to the manufacture of sintered Nd-Fe-B magnets at the sacrifice of the advantage of using inexpensive source materials. The powder metallurgy process includes a step of finely dividing a Nd-Fe alloy ingot to a size of from about 2 to about 10 .mu.m. This step is difficult to carry out because the Nd-Fe alloy ingot is readily oxidizable. In addition, the powder metallurgy process requires a number of steps including melting, casting, rough crushing of ingot, fine crushing, pressing, and sintering until a magnet is completed.
On the other hand, the rapid quenching process is advantageous in that a magnet can be produced by a rather simple process without a fine pulverizing step. The rapid quenching process requires a smaller number of steps including melting, rapid quenching, rough crushing, and cold or hot pressing until a magnet is completed. Nevertheless, coercive force, energy product, and magnetizing behavior must be improved as well as cost reduction before rapidly quenched magnets can be commercially acceptable.
Among the properties of rare earth element-iron-boron permanent magnets, coercivity is sensitive to temperature. Rare earth element-cobalt magnets have a temperature coefficient of coercive force (iHc) of 0.15%/.degree.C., whereas rare earth element-iron-boron magnets have a temperature coefficient of coercive force (iHc) of 0.6 to 0.7%/.degree.C., which is at least four times higher than the former. The rare earth element-iron-boron magnets have the likelihood of demagnetizing with an increasing temperature, limiting the design of a magnetic circuit to which the magnets are applicable. In addition, this type of magnet cannot be incorporated in parts which are mounted in an engine room of automobiles used in the tropics.
As is known in the prior art, a high temperature coefficient of coercive force creates a bar when it is desired to commercially use rare earth element-iron-boron permanent magnets. There is a need for development of a magnet having a great magnitude of coercive force (see Nikkei New Material, 4-28, No. 9 (1986), page 80).
Japanese Patent Application Kokai No. 60-9852 or Croat, EPA 0108474 describes how to impart high values of coercive force (iHc) and energy product to R-Fe-B alloy by rapid quenching. The composition is claimed as comprising at least 10% of rare earth element of Nd or Pr, 0.5 to 10% of B, and a balance of Fe. It was believed that the outstanding magnetic properties of R-Fe-B alloy were attributable to the Nd.sub.2 Fe.sub.14 B compound-phase. Accordingly, regardless of whether the method is by sintering or by rapid cooling, most prior art proposals for improving magnetic properties were based on experiments using materials having a composition in proximity to the above compound, i.e., 12-17% of R and 5-8% of B (see Japanese Patent Application Kokai Nos. 59-89401, 60-144906, 61-79749, 57-141901, and 61-73861).
Since the rare earth elements are expensive, it is desired to reduce their content as low as possible. Unfortunately, coercive force (iHc) is dramatically reduced at a rare earth element content of less than 12%. As indicated in FIGS. 11 and 12 of EPA 0108474, iHc is reduced to 6 kOe or less at a rare earth element content of 10% or less. Although it is known for R-Fe-B alloys that coercivity is reduced at a rare earth element content of less than 12%, no method is known for controlling the composition and structure of an R-Fe-B alloy so as to optimize magnetic properties while preventing coercivity from decreasing.
Although Nd.sub.2 Fe.sub.14 B compound is used as the basic compound in both the sintering method and the rapid quenching method, the magnets produced by these methods are not only different in the production method, but also belong to essentially different types of magnet with respect to alloy structure and coercivity-generating mechanism, as described in Oyobuturi (Applied Physics), Vol. 55, No. 2 (1986), page 121. More particularly, the sintered R-Fe-B magnet has a grain size of approximately 10 .mu.m and is of the nucleation type as observed with SmCo.sub.5 magnet in which coercivity depends on the nucleation of inverse magnetic domains, if compared to conventional SmCo magnets. On the contrary, the rapidly quenched magnet is of the pinning type as observed with Sm.sub.2 Co.sub.17 magnet in which coercivity depends on the pinning of magnetic domain walls due to the extremely fine structure of fine particles of from 0.01 to 1 .mu.m in size being surrounded by an amorphous phase which is richer in Nd than Nd.sub.2 Fe.sub.14 B compound (see J. Appl. Phys., 62(3), Vol. 1 (1987), pages 967-971). Thus any approach for improving the properties of these two types of magnets must first take into account the difference of coercivity-generating mechanism.
We have proposed in Japanese Patent Application No. 62-90709 a permanent magnet having a composition of R.sub.x T.sub.(100-x-y-z) B.sub.y M.sub.z wherein 5.5.ltoreq.x.ltoreq.20.0 and R, T, y and z have the same meanings as defined in the present disclosure, having a fine crystalline phase or a mixture of a fine crystalline phase and an amorphous phase. This magnet is still not fully satisfactory.